July 2005
Audiopax Stereo
Eighty Eight Stereo Amplifier: Measurements
All amplifier measurements are performed
independently by BHK Labs. Please click to learn
more about how we test amplifiers there. All measurement data and graphical
information displayed below are the property of SoundStage! and Schneider
Publishing Inc. Reproduction in any format is not permitted.
 Measurements were made at 120V AC line voltage with both
channels being driven. All measurements made with "timbre" control in mid
position unless otherwise noted.
 Gain: 4.25x, 12.8dB.
 Output noise, 8ohm load, unbalanced input, 1kohm input
termination: wideband 0.292mV, 79.7dBW; A weighted 0.040mV, 97.0dBW.
 AC line current draw at idle: timbre control
counterclockwise, 0.86A; timbre control at mid rotation, 1.0A; timbre control clockwise,
1.37A.
 Output impedance at 50Hz: 3.1 ohms.
 This amplifier inverts polarity.
Power output with 1kHz test signal
 8ohm load at 1% THD: 0.75W
 8ohm load at 10% THD: 14.6W
 4ohm load at 1% THD: 0.17W
 4ohm load at 10% THD: 8.7W
General
The Audiopax Stereo Eighty Eight is a low/mediumpowered
singleended tube amplifier utilizing a single KT88 output tube in each channel. The
design is unusual in that it has a control for adjusting the "timbre" of the
sound for different speakers and situations. This control appears to vary the quiescent
current in the output tube. It turns out that one measurable effect of this control is to
vary the damping factor of the output. Gain and distortion characteristics are also
affected. Gain of the unit is somewhat lower than usual for power amplifiers. Rated power
is 15W into an 8ohm load.
Chart 1 shows the frequency response of the amp with
varying loads. As can be seen, the output impedance, as judged by the closeness of spacing
between the curves of open circuit, 8ohm, and 4ohm loading, is of a typical value for
tube amplifiers. The variation with the NHT dummy load in the audio range is on the order
of +/2dB.
Chart 2 illustrates how total harmonic distortion plus
noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. This
design, with its single output connection for speaker load, puts out about the same power
into 4 or 8ohm loads, although the distortion is greater with 4ohm loading.
Total harmonic distortion plus noise as a function of
frequency at several different power levels is plotted in Chart 3. The amount of rise in
distortion at high frequencies is admirably low, but the increase in distortion at low
frequencies is quite pronounced.
As an illustration of one of the parameters that are
affected by the timbre control, damping factor is plotted in Chart 4 as a function of the
timbre control set at counterclockwise, midrotation, and clockwise positions. This
measurement was done at a much lower current (.125A) than the usual 1A in order to
maintain signal linearity at the lowest timbre setting.
A spectrum of the harmonic distortion and noise residue of
a 10W 1kHz test signal is plotted in Chart 5. The magnitude of the ACline harmonics is
very low, and the signal harmonic spectrum has the desirable characteristic of tapering
off as the harmonic number increases.
Chart 1
 Frequency Response of Output Voltage as a Function of Output Loading 
Red line: open circuit
Magenta line: 8ohm load
Blue line: 4ohm load
Cyan line: NHT dummyspeaker load
Chart 2  Distortion as a Function
of Power Output and Output Loading 
(line up at 2W to determine lines)
Top line: 4ohm SMPTE IM
Second line: 8ohm SMPTE IM
Third line: 4ohm THD+N
Bottom line: 8ohm THD+N
Chart 3  Distortion
as a Function of Power Output and Frequency 
8ohm output loading
Cyan line: 14W
Blue line: 10W
Magenta line: 5W
Red line: 1W
Chart 4  Damping Factor
as a Function of Frequency 
Damping factor = output impedance divided into 8
Red line: timbre control fully clockwise
Magenta line: timbre control at mid position
Blue line: timbre control fully counterclockwise
Chart 5  Distortion and
Noise Spectrum 
1kHz signal at 10W into an 8ohm load
